Method for making a detachable electrical contact

ABSTRACT

An electrical interconnection, which includes a method for fabricating the device, is disclosed. The interconnection comprises two contact surfaces, on at least one of which is disposed at least one solid metal conical projection in predetermined dimension and location. Rather than necessarily being permanently cojoined, the contact surfaces are attachable and detachable when desired. The conical projections on one contact surface make ohmic contact, either by wiping with an intermeshing like structure on a second contact surface or by contacting a second contact surface which is a substantially flat contact pad. An interconnection, in this invention, is the combination of at least one contact having individual conical projections and another contact, optionally having individual conical projections. The conical projections are formed in metal by electrochemical machining in neutral salt solution, optionally in a continuous foil. The conical projections are also optionally formed on the head of a contact pin.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrical interconnection and meansof making the interconnections, which are useful in electronic packagingapplications such as in semiconductor integrated circuit chips andcircuit boards and cards, cables and modules.

Recent developments in integrated circuits have clearly demonstrated thebenefits which can be achieved by fabricating electrical devices intosmaller and smaller packages. These small packages are densely packed,being multilevel with signal and power planes and other features onvarious levels and means of interconnecting selected levels to oneanother. The interconnections themselves provide sites for potentialsignal degradation. For example, interconnections between levels ofconductor lines, and between conductor lines of a printed circuit board(PCB) or card and any electrical devices mounted thereon can be madebetween conductive areas called pads. Impedance matching, minimum numberof discontinuities and redundancy must be present at theseinterconnections in order to permit rapid, low noise, low loss, lowresistance signal transmission. Approaches used at present in devisinginterconnections for these electronic packages may require numerousprocessing steps, and the attachment of surface mounted devices mayrequire soldering and rework, often involving exposing the components todestructive temperature cycling.

Copending application SN 07/520,335, filed May 7, 1990 to Burns et al.and commonly assigned with the present invention describes a single anda double sided contact comprised of polymeric cones which have beenformed in polymer sheets by excimer laser and have been surfacemetallized. The cone connectors of the present invention, like that inthe copending application, provide the low cost, high frequency,redundant, fine tipped high performance pad-on-pad contacts required.However, the cones of the present invention, being differently comprisedand differently fabricated, provide an alternative to those of thecopending application.

2. Description of the Art

Electrical interconnections comprised of interdigitated dendriticprojections are a fertile field of scientific inquiry. The conicalprojections of the present invention are distinguished from dendriticprojections by the method of making, by composition, and by thecontrolled location and dimensions of conical projections. Schemesproposed to strengthen dendrites, such as coating with soft metal aredescribed in IBM Technical Disclosure Bulletin, Vol. 22, No. 7, p. 2706by Babuka et al and copending application Ser. No. 07/415,435 to Cuomoet al., filed Sept. 28, 1989 and commonly assigned with the presentinvention.

IBM Technical Disclosure Bulletin Vol. 22, No. 7, p. 2706, publishedDecember, 1979 by Babuka et al. describes a high density pad-to-padconnector on which dendrites are grown on a pad and coated with a liquidgallium alloy. When the dendritic pad is mated, the dendrites pierce thetarnished liquid metal film of a second pad and make the electricalcontact.

IBM Technical Disclosure Bulletin, Vol. 24, No. 1A, June, 1981, p. 2,"Process For Producing Palladium Structures" by Armstrong et aldescribes that the small cross-section of the base of the dendrite is atleast partly responsible for breakage of dendrites. It also describesthe need for "wipe" to make low resistance contact, but states that thesufficient wipe.

IBM Technical Disclosure Bulletin, Vol. 23, No. 8, January, 1981, p. 1,"Dendrite Connector System With Reinforced Base" by Armstrong agreeswith the above diagnosis, but differs in the proposed cure, proposinginstead reflowing tin around the bases of the dendrites. Dendrites aspad-to-pad contact elements are also described in Research Disclosure,March, 1988, No. 287, p. 28748, "Method to Provide Multiple DendriticContact Points for High Density Flat on Flat Connector System",disclosed anonymously. Again, the dendrites, described are irregularlyshaped and randomly located. However, the reduced connector length ofthe dendrites are described as providing noise reduction and improvedsignal speed, and the references suggests that having multiple contactpoints lowers contact resistance.

The cones of the present invention, unlike the dendrites of several ofthe above references, do not require reinforcement.

Other means in the art of making electrical interconnection betweencontact pads include spheres (U.S. Pat. No. 3,634,807 issued Jan. 11,1972 to Grobe et al, U.S. Pat. No. 4,604,644 issued Aug. 5, 1986 toBeckham et al) conductive rods (U.S. Pat. No. 4,644,130 issued Feb. 17,1987 to Bachmann, U.S. Pat. No. 4,050,756 issued Sept. 27, 1977 toMoore, and U.S. Pat. No. 4,240,198, issued Dec. 23, 1980 to Alonso),hollow posts (U.S. Pat. No. 3,725,8545) and third structures interposedbetween and parallel to the connector pads but separate from both (U.S.Pat. Nos. 3,881,799, issued May 6, 1975 to Elliott et al and 3,634,807,issued Jan. 11, 1972 to Grobe et al).

Flat-topped protrusions, permanently connecting pads between levels in amultilayer structure are described in the art (U.S. Pat. No. 4,751,563,issued June 14, 1988 to Laibowitz et al).

U.S. Pat. No. 3,634,807, issued Jan. 11, 1972 to Grobe et al. describesa removably attachable contact comprising a plurality of hollow metalspheres or wire balls mounted in a predetermined pattern on either sideof a flexible insulating sheet. Alternatively, metal is deposited inopenings at the intersection of thin strips of insulating material. Inanother embodiment, a conductive sheet is sandwiched between sets ofcontact elements. These embodiments are designed to be relativelyinflexible in the X-Y direction and flexible in the Z direction.

U.S. Pat. No. 3,725,845 issued Apr. 3, 1975 to Moulin describes ahermaphroditic connector comprising a plurality of hollow posts. It is alarge scale connector for watertight use with cables in geophysicalsurveying, rather than for use with microminiature contact pads inpackaging.

U.S. Pat. No. 3,881,799 issued May 6, 1975 to Elliott et al. describes aconnector that comprises a plurality of domes projecting from both sidesof a spring matrix, interposing a third element between the contacts tobe connected, the third element being integral to neither.

All the above nondendritic contact means are inadequate for use in highpacking density structures, being of dimensions which are too large andtoo vulnerable to dirt contamination.

U.S. Pat. No. 4,644,130 issued Feb. 17, 1987 to Bachmann describes aplurality of elastomeric connector rods which have been renderedconductive by being filled with conductive particles dispersed therein.

U.S. Pat. No. 4,751,563 issued June 14, 1988 to Laibowitz et al.describes a method of making a cone shaped structure, having acarbonaceous surface contaminant, using an electron beam. A conductivelayer is deposited on at least a portion of the cone and over thesubstrate area around the base of the cone. Then an insulating materialis applied overall and any further processing is performed. Structuresdescribed in this patent are in the nature of through-holes, buriedirreversibly within a unitary multilayer structure rather than beingremovably attached. Since electron beam radiation is used, the materialfrom which the cone is comprised must of course be removable by electronbeams.

Unlike connectors described in the art, the electrochemically machined(ECM) connector of the present invention is simply fabricated,reproducible, completely metallic, substantially smooth, of a singlepreselected height, non-brittle and applicable to interconnection ofhigh density circuitry. Furthermore, the neutral salt electrolytesolutions in which the cones of the present invention are prepared poseno known safety problem, even when recirculated as in the presentinvention.

A number of fundamental studies of electrochemical machining have beenreported in journal literature. The importance of mass transportconditions for high rate dissolution of iron and nickel in 5M NaCl, 5MNaClO3 and 6M NaNO3 was discussed in "On the Role of Mass Transport inHigh Rate Dissolution of Iron and Nickel in ECM Electrolytes" Parts Iand II, by Datta et al. in Electrochimica Acta, Vol. 25, pages1255-1263, 1980. Anodic levelling of nickel peaks in NaCl solution wasdiscussed in "On the Theory of Anodic Levelling: Model Experiments withTriangular Nickel Profiles in Chloride Solution" by Clerc et al. inElectrochimica Acta, Vol. 29, pages 1477-1486, 1984. More recently,micromachining of small dimensions ranging from several microns tomillimeters has been discussed for various metal andsemiconductor-electrolyte systems in "Application of Chemical andElectrochemical Micromachining in the Electronics Industry", by Datta etal. in Journal of the Electrochemical Society, Vol. 136, No. 6, pages285C-292C, June, 1989. Drilling of holes and slots in nickel and steelin neutral salt solutions is discussed in "Jet and Laser-JetElectrochemical Machining of Nickel and Steel" by Datta et al. inJournal of the Electrochemical Society, Vol. 138, No. 8, pages2251-2256, August 1989. The feasibility of etching grooves in stainlesssteel in neutral salt solutions through photoresist masks, and problemsinherent in the process were discussed in "Electrochemical Dissolutionof Stainless Steels in Flow Channel Cells With and Without PhotoresistMasks" by Rosset et al., Journal of Applied Electrochemistry, Vol. 20,pages 69-76, 1990.

These articles in general report on fundamental studies ofelectrochemical machining which have been performed under controlledhydrodynamic conditions in order to acquire an understanding of theanodic behavior of the metal-electrolyte system. Electrochemicalmachining involves a high rate of metal removal from a workpiece thathas been made anodic in an electrolytic cell. In neutral salt solution,hydrogen evolution takes place at the cathode. The process of metalremoval from the anode, being electrochemical in nature, is independentof the hardness of the metal to be removed. Unlike mechanical machining,the ECM process does not introduce stresses into the machined workpiece.

Based on electrochemical studies, ECM systems are divided into twotypes: passivating and non-passivating. Passivating electrolytes containoxidizing anions, such as nitrate anions, resulting in the formation ofan oxide film on the anode material and possible oxygen evolution ratherthan metal dissolution at the anode at low current densities. At highcurrent densities, however, a high rate of metal dissolution is possiblein oxidizing systems. In non-passivating electrolytes, because of thepresence of aggressive anions, oxide films do not form and oxygenevolution is not possible. Metal dissolution is the only anodic reactionin the non-passivating electrolyte. In conventional ECM, in whichphotolithographic masking is not used, the passivating electrolytes aregenerally preferred because of their inert nature and their ability toevolve oxygen at low current densities, thereby minimizing stray cuttingeffects. However, for the present invention, in which masking isemployed, oxygen evolution may cause lifting or may otherwise damage themask.

In both passivating and non-passivating systems, the rate of metaldissolution is dependent on the current density (or on the appliedvoltage), electrolyte concentration, and to an extent on thehydrodynamic conditions.

None of the journal articles summarized above describes the fine tippedpad-to-pad cone connectors of the present invention nor the manner ofmaking.

Thus, it is one object of the present invention to use anelectrochemical machining technique in a simple salt solution to providehigh performance solid metal cone connectors.

It is a further object of the invention to provide the capability tointerconnect high density packages of electrically mounted devices andPCBs and/or cards to each other and to cables.

It is a further object of the invention to provide an electricalinterconnection which permits reliable, rapid, dirt tolerant, low noise,low loss, low resistance signal transmission.

It is a further object of the invention to provide a method of makingthe electrical interconnection described above in an efficient andcontrollable manner.

It is a further object of this invention to provide a conical electricalinterconnection useful in the art of electronic packaging.

It is a further object of the invention to provide a low resistanceelectrical interconnection nondestructively connectable anddisconnectable. consisting of solid metal cones.

Still another object of the invention is to provide a fabrication methodto produce an electrical interconnection between two contact surfaces,at least one of which comprises essentially perpendicular conicalprojections of predetermined pattern and dimensions.

These and other objects, features and advantages of the presentinvention will become more apparent from the descriptions to follow.

SUMMARY OF THE INVENTION

The electrical interconnection of the present invention comprises afirst and a second contact surface, at least one of which includes aconductive substrate having conical protrusions extending substantiallyperpendicularly therefrom. The conical conductors comprise a pluralityof solid metal cones suitable for high performance electrical contact,as described more fully hereinbelow.

The metal cones are formed by high speed electrochemical machining of ametal foil, sheet, pin head or the like through a mask. The processcomprises high speed electrochemical etching in an aqueous salt solutionby selective anodic dissolution of areas defined on the metal by a mask.The process is applicable to a wide variety of metals and alloysincluding, but not limited to, copper, gold, chromium, tin, lead,nickel, aluminum, titanium, rhodium, palladium and steel, and isindependent of the hardness of the etched material.

As in the aforementioned copending application, the present inventionincludes a single and a double sided embodiment.

A stream of aqueous salt solution is directed at the metal surface,which is anodically charged. Metal is removed from areas not blocked bythe mask. The arrangement of photoresist dots on the mask predeterminesthe location of the conical projections to be formed. A secondconductive surface is optionally also provided with conical projectionsdesigned to wipe and intermesh with those on the first conductivesurface and prepared in the same manner. The conductive cones extendingsubstantially perpendicularly from their respective conductivesubstrate, contacted together, form the interconnection, which providescontact while being freely attachable and detachable as desired, forexample, for testing. Alternatively, they can be permanently cojoined bysoldering or the like.

In the double-sided embodiment, wherein conical projections disposed ona first conductive substrate are brought into interdigitated contactwith like conical projections similarly disposed on a second conductivesubstrate, the respective spacing of the conical projections is suchthat there is mutual "wipe" between contacting conical projectionswithout their breakage, while at the same time surface contaminants aredisplaced from the conical projections.

Contact is further facilitated by the shaping of the top of each of theconical projections, which are sharply domed, and by the fact that inoperation a typical conical projection is in contact with four nearestneighbor cones.

In the single-sided embodiment, conductive conical projections, disposedon only a first conductive surface, are brought into contact with asecond, planar, conductive surface, forming the electricalinterconnection. The tips of the conical projections are brought intocontact with enough force to displace any contaminants which may bepresent on the second conductive surface.

Alternatively, in a pin-type conical interconnection, a secondconductive surface may, instead of contact pads alone, includeconductive through-holes or blind vias, within which contact is made bya connector pin. The pin is optionally soldered in place. Then theconical projections, which are disposed on the head of the pin, arebrought into contact with a first conductive surface.

The conical projections are essentially perpendicular in relation totheir respective conductive surface. The height of the cones is selectedin part to be sufficient for any contaminant or dust to residetherebetween when displaced such that the resistance of theinterconnection to be formed is not elevated thereby. Normally in thepresent invention cones of about 60 to about 80 microns high wereprepared and found to be of adequate height.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical photograph of an unetched metal substrate maskedwith an array of photoresist dots.

FIG. 2 is a schematic representation of the formation of cones duringthe electrochemical machining process through a mask.

FIG. 3, a b and c show scanning electron micrographs (SEMs) of thefabrication by electrochemical machining through a mask of metal conescomprised of (a) hardened stainless steel, and (b, c) copper.

FIG. 4 is a schematic representation of the steps involved in a roll toroll process of making metal cones.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The array of conical projections comprises metal conical protrusionsformed by electrochemical machining in a neutral salt solution through apolymer mask. The process involves the principles of high speeddissolution of metals. In order to fabricate cones on metal films,foils, sheets or pin heads, through-mask machining concepts areemployed. Photoresist is applied to the metal material. Photoprocessingleaves evenly spaced dots suitably arrayed for the pad-on-pad connector(FIG. 1). It will be obvious to one skilled in the art that alternatemeans other than photoprocessing may be used to create the dots, such assilk-screening. The metal substrate masked with photoresist is madeanodic in an electrolytic cell, where it directly and closely faces acathode. An electrolyte flows through the spacing between the cathodeand the anode. Upon passage of the current, the anode material dissolvesin the areas which remain unprotected by the photoresist. As anodicdissolution continues, the removal of material between the dots leads toformation of cavities which, under certain conditions, growpreferentially in the vertical direction, seen in FIG. 2, finallyleading to the formation of the cones seen in FIG. 2. Preferentialdissolution in the desired direction is a key to the success of theprocess. Preferential dissolution in the desired direction is achievedby employing an impinging multi-nozzle flow or, alternatively, byemploying a channel flow apparatus so that extremely high electrolyteflow rate can be provided. If the system is run with a slow, i.e.laminar, flow, cone formation and preferential etching will not occur.In addition to the electrolytic flow cell and means for providingextremely high flow rate, the apparatus used also comprises a smallcentrifugal pump, and a filtering unit to filter out the hydroxideprecipitates that are formed in the solution as the machining proceeds.

Using neutral salt solutions comprising sodium nitrate or,alternatively, sodium chloride, cones can be fabricated in both copperand in hardened stainless steel by through-mask electrochemicalmachining at extremely high speed. In order to obtain best results it isnecessary to choose a metal-electrolyte combination and operatingparameters which yield high current efficiency for metal dissolution,because the anodic evolution of oxygen leads to premature detachment ofthe photoresist dots and consequent unsatisfactory cone formation.

FIG. 3 shows SEMs of some of the cones fabricated on copper film and onstainless steel. It will be clear to one skilled in the art that ajudicious selection of electrolyte and machining conditions will permitfabrication of cones from other metals and alloys as well.

FIG. 4 describes in a flow chart a continuous system for large scalereel-to-reel cone formation in a manufacturing environment.

The process described can be used in a continuous flow manufacturingoperation, wherein the metal anode overlaid with the photoresist patternpasses at high speed through a set of equally spaced, multi-nozzledcathodes through which an electrolyte flows. In order to obtainuniformly distributed, needle-like cones, current distribution problemsat the anode can be solved by the addition of dummy photoresist linessurrounding the area where cones are to be formed, a technique known inthe electrochemical art.

For some applications, an optional additional step of overcoating thefinished cones with noble metals, such as by electroplating or byelectroless plating about 50 to about 100 microinches of hard gold onstainless steel cones, may be desired in order to maintain low contactresistance. If the cones were comprised of copper, the fact of gold andcopper interdiffusion would suggest a diffusion barrier of about 50 toabout 100 microinches of nickel between the copper cones and the goldovercoat.

Metal discs fabricated by this method can be reflow soldered ontocircuit pads on flexible circuits, circuit boards or cards with coneside up for making pad-to-pad contact.

It should also be noted in general that the narrower the tips of thecones, the better the contact that can be made. However, the broader thebase, the more resistant are the cones expected to be to lateral forces.

EXAMPLE 1

An ECM electrolyte consisting of Molar NaCl was prepared to be used atambient temperature. The anode material was hardened, Number 420 (Fe13%Cr) stainless steel sheet, 1/8 inch thick. The stainless steel anode wascleaned by conventional degreasing methods using isopropyl alcohol.Dynachem UF photoresist was applied in the manner recommended by themanufacturer and a pattern of photoresist dots was developed on theanode surface. Any liquid or dry photoresist that can withstand a pHenvironment of about 3 to about 8 can be used. The samples, withphotoresist masks, were mounted on an anode holder and placed in anelectrolytic cell where a shower of electrolyte was directed to thesample anode. Any typical electrolytic cell can be used or adapted. Thecathode assembly consisted of a perforated stainless steel plate actingas the shower head which was mounted on a Plexiglas (trademark of Rohmand Haas Co.) enclosure. The electrolyte from a reservoir was pumpedthrough the cathode assembly. The rate of the electrolyte flow was 3gallons per minute (gpm). The spacing between the cathode plate and theanode was 3 mm. This arrangement provided extremely high electrolyteimpingement at the sample anode, thus leading to preferential metalremoval from the direction that is required to obtain cones. A potentialof 5 volts was applied across the cell. Under these conditions oxygenevolution did not occur and very high rate of material removal wasobtained. The amount of electrical charge necessary to obtain a desiredcone shape was measured by a coulometer. The coulometer was also used asa means of determining the end point of the ECM operation.

As shown in FIG. 2, formation of cones with sharp needles requiredremoval of material to be continued until the photoresist undercuttingwas nearly half the diameter of the photoresist dots. The amount ofelectrical charge necessary to obtain a desired cone shape is dependenton the dot size and the undercutting, which in turn is dependent on themetal removal rate and the impingement rate of the electrolyte. Optimumconditions leading to desired cone shapes can, in principle, be obtainedby mathematical modeling of the system. We have determinedexperimentally the amount of charge necessary to determine the needleshapes for the above parameters. We found that for a dot size of 4 mils(100 microns), a material removal of about 60 microns thick wasrequired. The amount of charge required is related to the thicknessremoved by Faraday's Law which can be written as:

    Delta 1 = QM/nF rho

where Delta 1 is the thickness of the material removed, Q is thequantity of charge, M is the molecular weight, n is number of electronstransferred (= 3.4 for FE13% Cr stainless steel), the F is Faraday'sconstant and rho is the density of the material. The amount of chargewas monitored in a coulometer. By varying the amount of charge, cones ofdifferent heights can be obtained.

EXAMPLE 2

The details of the procedure used were substantially similar to thatdescribed in Example 1, except that the anode material consisted ofcopper plates. The cleaning procedure consisted of dipping in diluteapplication. The applied voltage was 3, and in the above mentionedFaraday equation n=2.

EXAMPLE 3

The electrochemical micromachining method of making metallic coneconnectors can be easily adapted to continuous flow manufacturing on aroll to roll process. The process steps that would be involved areindicated in the schematic shown in FIG. 4.

It will be understood that the present invention may be embodied inother specific forms and processes without departing from the spirit oressence thereof. The present examples and embodiments are to beconsidered as illustrative rather than restrictive, and the inventionunlimited to the details recited herein.

We claim:
 1. A method for making a detachable electrical contact on aconductive surface comprising the steps of:applying a nonconductivematerial in a preselected array of polymer dots on the conductivesurface; and electrochemically machining the conductive surface until asolid metal cone is formed beneath each of the polymer dots, a dot beingat the apex of each cone and a continuous metal remaining toelectrically connect each cone to each other cone at their bases.
 2. Themethod recited in claim 1, wherein the step of applying a nonconductivematerial comprises applying a photoactive polymer and exposing anddeveloping the polymer to form a predetermined array of polymeric dots.3. The method recited in claim 1, wherein the step of electrochemicallymachining in solution comprises electrochemically machining in asubstantially neutral salt solution.
 4. The method recited in claim 1,wherein the step of applying a coating of nonconductive material ontothe conductive surface comprises applying a coating of nonconductivematerial onto a conductive surface comprised of a material selected fromthe group consisting of copper, gold, chromium, tin, lead, titanium,nickel, aluminum, rhodium, palladium, steel; of the aforelisted.
 5. Themethod recited in claim 1 wherein the step of electrochemicallymachining the conductive surface comprises electrochemically machining acontinuous metal foil which is moved continuously into, through and outof solution at a preselected rate.